In the case of rotary and percussive drilling of blast holes in rock, hard material drill heads are connected non-rotatingly with a hollow drill shaft, which is driven at least partially rotatingly and percussively by a shank end of a drill tool.
For force-form fitting connection between the tool end of the drill shaft and the drill head, the drill shaft has a conical surface and a cup-shaped drill head has an associated inner conical surface. The cup base of the drill head is especially subjected to high alternating stresses by virtue of the intensive percussive stress developed via the included angle of the cup base, which in local areas of high stress intensities can result in material fatigue and ultimately in failure.
U.S. Pat. No. 4,716,976 discloses a cup-shaped drill head wherein the extension of the inner conical surface on the leading end has an inner cylindrical surface having a zero curvature in longitudinal cross-section and a connecting mathematically smooth inner spherical cap as the base of the cup, whose uniform curvature corresponds to the reciprocal value of the inner cylindrical radius. The mathematically smooth curvature that is the angle-free change over from the inner cylindrical surface to the inner spherical cap results at this location in a locally increased material stressing, which as van Mieses failure criteria, for example, consequently defines the load limit of the drill head.